1. Field of the Invention
The present invention relates to a fuel cell system for generation of electric energy by reaction between a hydrogen-rich fuel gas and oxygen gas.
2. Description of Related Art
As is widely known, a fuel cell is continuously supplied with a fuel (hydrogen) and an oxidizing agent (oxygen) from outside of the fuel cell when generating electricity. The fuel is supplied to a negative electrode side whereas the oxidizing agent is supplied to a positive electrode side of the cell. The positive electrode and the negative electrode are separated from each other by an electrolytic member. The fuel cell converts chemical energy generated in oxidization of the supplied fuel directly into electric energy, serving as an electric power source.
An operating principle of the fuel cell is as follows: First, the hydrogen supplied as the negative-electrode active agent dissociates into electrons and protons. In order to promote the dissociation, platinum (Pt) for example is used as a catalyzer. After the dissociation, the protons pass the electrolyte of the fuel cell, and react with the supplied oxygen (positive-electrode active agent). The reaction yields water. On the other hand, the electrons resulted from the dissociation moves from the negative electrode to the positive electrode, generating electromotive force between the two electrodes.
The fuel cell directly converts chemical energy into electric energy. In this method, higher conversion efficiency can be expected than in a thermal generation method. For this reason, the fuel cell can be utilized effectively as a power source for a drive motor of an electric car for example. Further, exhaust from the fuel cell is mainly water vapor, which is not toxic like carbon monoxide contained in exhaust from an internal combustion engine.
Normally, the fuel cell does not function by itself as a source of electric supply. In order for the fuel cell to operate, means for supplying hydrogen gas to the fuel cell and other peripheral devices must be deployed. In other words, the fuel cell is part of a fuel cell system further comprising other peripheral devices, and operates within this system. The means for supplying the fuel cell with hydrogen may be pressure feeding from a high-pressure container loaded with hydrogen, or pressure feeding of hydrogen gas obtained by reforming a hydrogen-containing substance. Using the hydrogen gas stored in the high-pressure container can relatively simplify the system, and has a number of other advantages. On the contrary, a very high pressure and a long time are required for charging the container with hydrogen. Another problem is that a specific infrastructure such as a gas charge station network has to be prepared. These problems have been a major hurdle for making practical an electric automobile powered by the fuel cell.
The above problem can be solved by the feeding of hydrogen gas obtained by reforming a hydrogen-containing compound (such as methanol). Publicly known examples of this method include contacting the hydrogen-containing compound with steam, and oxidizing part of the hydrogen-containing compound. In these methods, the reformation of the hydrogen-containing compound leaves carbon dioxide and a trace quantity of carbon monoxide gas. Therefore, the desired hydrogen gas is obtained as a gas mixture containing these carbon dioxide and carbon monoxide gases.
If the platinum is used as the catalyzer for the negative electrode, the carbon monoxide contained in the gas mixture is problematic in the following point: As is widely known, platinum is poisoned by carbon monoxide gas, and gradually deteriorates in its activity. Therefore, if the hydrogen gas is supplied to the fuel cell as the gas mixture containing the carbon monoxide gas, life of the platinum catalyzer is shortened.
The poisoning of the platinum can be eliminated by converting the carbon monoxide gas in the gas mixture into carbon dioxide gas, and then supplying this modified gas mixture to the fuel cell. Generally, a multi-stage conversion method is used in which the carbon monoxide is first oxidized at a high temperature, and then residual carbon monoxide is oxidized at a low temperature.
However, the fuel cell does not consume all of the supplied hydrogen gas for the generation of electricity. Part of the hydrogen gas is discharged out of the fuel cell as non-reacted gas. In an attempt to effectively use this non-reacted hydrogen gas, methods of recycling the discharged gas mixture (containing the non-reacted hydrogen gas and the carbon dioxide gas) from the fuel cell back into the fuel cell are being tried conventionally. However, concentration of the non-reacted hydrogen gas in the recycled gas mixture is lower than in the gas mixture originally supplied to the fuel cell. Therefore, in the method of reforming a hydrogen-containing compound in which obtained hydrogen gas is low in purity, repeated recycling of the gas mixture will enrich gases other than the hydrogen gas (such as carbon dioxide), unjustifiably decreasing energy conversion efficiency.
The present invention is proposed under the circumstances described above. An object of the present invention is to minimize the decrease in energy conversion efficiency of the overall fuel cell system, in a fuel cell system which generates electricity by using a gas mixture discharged from the fuel cell. Another object of the present invention is to provide a fuel cell stack suitably used in the above fuel cell system.
A fuel cell system provided by a first aspect of the present invention comprises a reforming device for producing a hydrogen-rich gas mixture by reforming a hydrogen-containing compound; a fuel cell for generating electromotive force by a reaction between hydrogen and oxygen; and further a hydrogen separating device disposed between the reforming device and the fuel cell. The hydrogen separating device is provided with hydrogen permeating means for obtaining a fuel gas by separating hydrogen gas from the gas mixture.
Preferably, the fuel cell system further comprises circulating means for supplying non-reacted gas discharged from the fuel cell, to the fuel cell as fuel gas.
The hydrogen-containing compound is one of ethanol, methanol, dimethyl ether, propane and natural gas.
The hydrogen permeating means may include a palladium alloy film. The palladium alloy film can be made of an alloy including palladium and at least one metal selected from silver, gold and ruthenium.
The hydrogen permeating means may include solid high-polymer hollow yarn. The solid high-polymer hollow yarn is made of polyimide for example.
The reforming device may include a steam generating portion for vaporization of water by heating, a combusting portion for heating the steam generating portion by burning a predetermined fuel, and a reforming portion for producing the hydrogen-rich gas mixture by reacting steam generated by the steam generating portion with the hydrogen-containing compound. The gas mixture from which hydrogen is separated by the hydrogen separating device can be used by the combusting portion as a fuel.
The fuel cell system may comprise a fuel cell stack including the fuel cell and at least an additional fuel cell laminated on the fuel cell.
A fuel cell system provided by a second aspect of the present invention comprises a plurality of fuel cell stacks each including a plurality of fuel cells in lamination, supply means for supplying a fuel and an oxidizing agent to each of the fuel cell stacks, and operation control means for controlling operation of the fuel cell stacks. The fuel cell stacks are divided into a plurality of groups including at least a first and a second groups, and the operation control means is arranged to operate and stop the fuel cell stacks of the first group independently of the fuel cell stacks of the second group.
Preferably, the operation control means is arranged to operate and stop each of the plurality of fuel cell stacks independently of the other fuel cell stacks.
Preferably, the fuel cell system further comprises piping for supplying the fuel to each of the fuel cell stacks, and a plurality of valves provided on the piping. The valves are arranged to supply and to stop the supply of the fuel to fuel cell stacks. Each of the valves is provided correspondingly to one of the plurality of groups.
Preferably, the operation control means controls open/close operation of the plurality of valves in accordance with an amount of electricity needed.
Preferably, the fuel cell system further comprises output control means provided with at least a pair of output terminals for receiving electricity from the fuel cell stacks and for outputting the electricity to an external component. The output control means is arranged to change a mutual connecting pattern among the fuel cell stacks, and a connecting pattern of the fuel cell stacks to the output terminals.
An automobile provided by a third aspect of the present invention comprises a drive motor and a fuel cell system for supplying electricity to the drive motor. The fuel cell system includes a plurality of fuel cell stacks each including a plurality of fuel cells in lamination, means for supplying a fuel and an oxidizing agent to each of the fuel cell stacks, and drive control means for controlling operation of the fuel cell stacks. The fuel cell stacks are divided into a plurality of groups including at least a first and a second groups. The drive control means is arranged to operate and stop the fuel cell stacks of the first group independently of the fuel cell stacks of the second group.
A fuel cell system provided by a fourth aspect of the present invention comprises a reforming device for producing a hydrogen-rich fuel-gas mixture by reforming dimethyl ether, and a fuel cell supplied with the fuel gas and an oxygen-containing gas, thereby causing a reaction between hydrogen gas and oxygen gas to produce electric energy and water. The reforming device includes a steam generating portion for vaporization of water, and a fuel reforming portion for performing steam reformation of the dimethyl ether. The dimethyl ether is heated, before being introduced into the fuel reforming portion, by heat generated at the steam generating portion.
At the fuel reforming portion, dimethyl ether is reformed into hydrogen and carbon dioxide by steam. Examples of a catalyzer disposed in the fuel reforming portion for promoting the reformation include Cu, for example, supported by an oxide of a base metal, a mixed (composite) oxide including one or more of Cr, Mn, Fe, Ni, Cu and Zn, and the mixed oxide supported by a heat-resistant support.
The fuel cell system further comprises a thermally conductive hollow member disposed in the steam generating portion. The dimethyl ether is introduced into the fuel reforming portion via the hollow member.
A fuel cell system provided by a fifth aspect of the present invention comprises a fuel cell, a hydrogen supplying source for supplying hydrogen gas to the fuel cell, and an oxygen-containing-gas supplying source for supplying an oxygen-containing gas to the fuel cell. The fuel cell system further comprises a water removing means for intermittently promoting removal of water remaining in the fuel cell.
Preferably, the water removing means is an electromagnetic valve provided between the oxygen-containing-gas supplying source and the fuel cell. Alternatively, the water removing means may be a pulsation pump provided between the oxygen-containing-gas supplying source and the fuel cell. Alternatively, the water removing means may be a supersonic generator.
The fuel cell includes a negative-electrode portion, a positive-electrode portion, an electrolyte portion disposed between the negative-electrode portion and the positive-electrode portion, a first plate disposed adjacent to the negative-electrode portion, and a second plate disposed adjacent to the positive-electrode portion. The first plate is provided with a hydrogen supplying groove for supplying the hydrogen gas to the negative electrode-portion. The second plate is provided with an oxygen supplying groove for supplying the oxygen-containing gas to the positive-electrode portion.
Each of the positive-electrode portion and the negative-electrode portion may include a catalyzer layer and a collector as a separate member from the catalyzer layer. In this case, the catalyzer layer is, for example, a porous member supporting a catalyzer. If the electrolyte portion is a solid member, the catalyzer layer may be formed by directly applying the catalyzer to a surface of the electrolyte.
Each of the positive-electrode portion and the negative-electrode portion may be an integration of the catalyzer layer and the collector.
According to a sixth aspect of the present invention, there is provided a method for removing unnecessary water remaining in a fuel cell supplied with a fuel gas and an oxygen-containing gas. The method comprises a step of supplying the oxygen-containing gas into the fuel cell, and a step of intermittently varying a pressure of the oxygen-containing gas.
Preferably, the pressure of the oxygen-containing gas is momentarily raised.
According to a seventh aspect of the present invention, there is provided a method for removing unnecessary water remaining in a fuel cell supplied with a fuel gas and an oxygen-containing gas. The method comprises a step of supplying the oxygen-containing gas into the fuel cell, and a step of applying high frequency vibration to the remaining water.
A fuel cell provided by an eighth aspect of the present invention comprises a negative-electrode portion for breaking hydrogen gas into hydrogen ions and electrons; a positive-electrode portion for producing water by a reaction of the hydrogen ions, electrons and oxygen gas; an electrolyte portion disposed between the negative-electrode portion and the positive-electrode portion in a manner allowing passage of hydrogen ions; a first plate disposed adjacent to the negative-electrode portion and provided with a hydrogen supplying groove for supplying the hydrogen gas to the negative-electrode portion; and a second plate disposed adjacent to the positive-electrode portion and provided with an oxygen supplying groove for supplying an oxygen-containing gas to the positive-electrode portion. At least one of the oxygen supplying groove and the hydrogen supplying groove is formed with a plurality of projections.
Preferably, the projections are provided with slopes for directing a flow of the supplied gas toward the electrolyte portion.
A fuel cell provided by a ninth aspect of the present invention comprises at least one electrode portion having a first surface and a second surface, an electrolyte portion adjacent to the first surface, and aplate adjacent to the second surface and formed with a gas supplying groove for supply of a predetermined gas. The gas supply groove is provided with a surface-pressure assuring means for assuring a surface pressure to the electrode portion.
Preferably, the plate is provided with a through hole communicating with the gas supplying groove. The surface-pressure assuring means is disposed near the through hole.
The surface-pressure assuring means includes a hollow member, and the hollow member has a thickness substantially equal to a depth of the gas supplying groove. Alternatively, the surface-pressure assuring means includes a gas-permeable porous member, and the porous member has a thickness substantially equal to a depth of the gas supplying groove.
The gas supplying groove may be provided with a stepped portion for supporting the surface-pressure assuring means.
According to the tenth aspect of the present invention, there is provided a plate-like separator used in a fuel cell stack comprising laminated fuel cells. The separator comprises a through hole for allowing passage of a supplied gas, a gas supplying groove communicating with the through hole, and a surface-pressure assuring means disposed in the gas supplying groove. The surface-pressure assuring means is placed near the through hole.
Other characteristics and advantages of the present invention will become clearer from detailed description to be made hereinafter with reference to the attached drawings.